Aircraft include flying machines fitted with a rotor connected to a carrier structure, e.g. an airplane fitted with a propeller or a rotorcraft fitted with a rotor for providing lift and possibly propulsion.
The rotor conventionally comprises a rotor mast secured to a hub, the hub carrying a plurality of radially-distributed blades.
Furthermore, the carrier structure possesses an airframe, sometimes referred to as a “fuselage”, having engine means arranged therein suitable for driving the rotor in rotation. In addition, the carrier structure includes a rotor mounting structure enabling the rotor to be fastened to the airframe.
Such a mounting structure usually includes mechanical transmission means and fastener elements for fastening the transmission means to the airframe. For example, a mounting structure of a rotorcraft known as a “pylon” comprises a main gearbox for transmitting power and means for fastening said main gearbox to the airframe, such as suspension bars, for example.
The engine means of the carrier structure then drive the rotor via the gearbox means of the mounting structure.
The carrier structure and the rotor are each subjected to forced excitations inherent to the speed of advance of the aircraft.
The dynamic excitation of the rotor, e.g. the lift and propulsion rotor of a helicopter, results from aerodynamic loads to which the rotor is subjected, these aerodynamic loads being resolved along stationary axes as a coplanar force acting in the general plane of the rotor hub, which plane is perpendicular to the axis of rotation of the rotor, an axial force that acts along the axis of rotation of the rotor, and a coplanar moment acting in a plane perpendicular to the axis of rotation of the rotor tangentially to the rotary movement of the rotor hub. The frequencies of such vibrations along axes that are “fixed”, i.e. tied to the airframe of the carrier structure, are equal to the product kbΩ, where “Ω” designates the speed of rotation of the rotor, “b” designates the number of blades, and “k” designates a positive integer. The fundamental frequencies correspond to the number “k” being equal to unity. These excitations are transmitted from the rotor to the structure via the pylon.
Similarly, the carrier structure is subjected to forced excitations. For example, the tail boom of a helicopter airframe is excited directly by a stream of turbulent air coming from the main lift and propulsion rotor.
Flying machines fitted with a rotor are generally structured to mitigate the consequences of such vibrations.
To this end, it is common practice to fit the rotor or the structure with anti-vibration systems, sometimes referred to as resonators, for filtering the dynamic forces at the frequencies that are the most troublesome, whether from the point of view of passenger comfort or from the need to avoid breaking an element that is subjected to such vibratory fatigue. These anti-vibration systems are then tuned to one of the harmonics of the speed of rotation of the rotor.
Under such conditions, proposals have been made, e.g. in document FR 2 808 256 (Eurocopter France), for pendular resonators in which the weight elements are mounted in pendular manner on the hub of the rotor head. Such pendular resonators serve to oppose the forced vibrations induced by the rotating rotor head by acting along the axis and in the plane of the rotor. The stiffness needed for such pendular resonators in order to filter the vibrations is provided by the centrifugal force field due to the rotation of the rotor. This rotation drives the pendular weight elements about hinges so that they perform harmonic motion at a frequency that is a multiple of the speed of rotation Ω of the rotor. The structure of such pendular resonators makes them easy to integrate in the rotor head.
Nevertheless, such pendular resonators are effective only at one given frequency. Such pendular resonators are arranged to be fitted to a rotor head located at the top of the carrier structure, the rotor providing lift and possibility also propulsion, and they do not take account of characteristics specific to the carrier structure, relating in particular to its mass and to its excitation frequency.
The effect of the resonators is to smooth vibration by creating anti-resonance at the given tuned frequency. Consequently, the resonator generates two new resonances (or vibration modes) at two respective frequencies that are situated on either side of the anti-resonance frequency. The frequency range defined by these two modes remains relatively narrow. Nevertheless, these two vibration modes created by the resonators are normally not troublesome insofar as both resonances differ from the given frequency that is to be filtered.
Vibration damper mechanisms are also known for damping the vibration that is the result of the forced excitation of the carrier structure. By way of example, reference may be made to document FR 2 784 350 (Eurocopter France) that describes a damped resonator arranged to be implanted in the tail of the carrier structure in order to filter given frequencies.
In addition to forced excitation, another vibratory phenomenon may give rise to major problems.
In the field of aviation in particular, a problem lies in attenuating vibratory phenomena induced by the aeroelastic instabilities to which a machine is subjected in flight. For example, such aeroelastic instabilities may result from coupling between the vibration modes of the carrier structure and the stream of air moving around it, i.e. in particular a fixed wing type structure (airplane wing) or a rotary wing type structure (rotor blades of a rotorcraft or airplane propeller). These instabilities are known to the person skilled in the art under the general term “flutter”.
Other aeroelastic instabilities correspond for example to the instabilities known as “whirl flutter” designating coupling between vibration modes of a rotor fitted with blades and vibration modes of the carrier structure supporting the rotor.
These phenomena of “flutter” and of “whirl flutter” are characterized by limit cycle vibration or by diverging vibration that can lead to breaking mechanical parts or structural elements. It is therefore essential to take these phenomena into account in the design of an aircraft in order to ensure that the critical speeds (forward speed, speed of rotation of the rotor) lie outside the limits of the flight envelope.
In particular, with whirl flutter, manufacturers ensure that rotor vibration modes do not couple with carrier structure vibration modes, thereby ensuring that those two assemblies are mutually compatible. In general, this can be done by appropriately placing the resonance frequencies and the respective dampers of the vibration modes of the various assemblies.
Nevertheless, it is difficult a posteriori to modify the modal characteristics of the carrier structure or of the rotor, should such phenomena appear during the development of an aircraft. Furthermore, a manufacturer may need to modify an existing aircraft in order to satisfy specific requirements of a user, and that may have an impact on the behavior of the aircraft when faced with such instabilities.
To solve these phenomena that may arise in the manner explained above, the manufacturer cannot make use of the resonators as described above that serve to filter forced excitations but not to filter couplings. Under such circumstances, the manufacturer often decides to modify the carrier structure, e.g. by making it stiffer, where such a modification involves not only a financial cost, but also an impact in terms of weight, neither of which are negligible. Furthermore, it may be difficult to make the modifications in question to an existing machine.
The state of the art also includes the following documents: EP 0 250 135, DE 10 2007 025934, FR 2 769 396, and FR 2 770 825
Document EP 0 250 135 describes a rotor carrier structure.
Document DE 10 2007 025934 describes a machine tool provided with a device including springs and dampers for eliminating given vibration.
Document FR 2 769 396 relates to a device for reducing noise in a cabin and not for avoiding destructive coupling between the vibration modes of a carrier structure and the rotor of the aircraft, that device including at least one sensor for measuring the values of a vibratory and/or acoustic parameter representative of a vibratory and/or acoustic effect of at least one noise source, mechanical means that are controllable and suitable for creating a force capable of reducing said vibratory and/or acoustic effect, and a control unit for controlling the mechanical means as a function of the values measured by the censor.
Document FR 2 770 825 seeks to reduce the vibration present in the cabin of a rotary wing aircraft by using a resonator arranged in the cabin.
Those documents therefore do not relate to solving a problem of coupling between vibration modes of a carrier structure and vibration modes of a rotor.